The present invention relates to a method for restoring a subterranean formation which may have become contaminated during an in situ leach operation and more particularly relates to a method of removing contaminants, i.e., ammonium ions, from a subterranean clay-containing formation after an in situ leach operation to restore the purity of any ground waters that may be present in the formation.
In a typical in situ leach operation, wells are completed into a mineral or metal value bearing (e.g., uranium) formation and a lixiviant is flowed between wells to dissolve the desired values into the lixiviant. The pregnant lixiviant is produced to the surface where it is treated to recover the desired values from the lixiviant. Unfortunately, many known, highly effective lixiviants not only leach the desired values from the formation but, also, they react with certain formations to give up chemical substances which remain in the formation after the lixiviants pass therethrough. Where the formation also contains ground waters and/or a water source which would otherwise be fit for human and/or animal consumption, these chemical substances will likely create a substantial contamination problem for this water. If this be the case, the formation must be treated after a leach operation to remove these contaminants to restore the purity of the water.
One method for improving the purity of a contaminated water source is to merely pump the water from the formation until the contaminant reaches an acceptably low level. Another, simple method is to pump uncontaminated water through the formation to flush out the contaminants. These methods work well where the contaminants are soluble and are not exchanged by some component of the formation from which it can only be released at a very slow rate. If the contaminants are exchanged by the formation, extremely large volumes of water must be used to adequately restore the formation.
In many known uranium and related value bearing formations, a substantial part of the formation matrix is comprised of calcium-based clays (e.g., smectite). This type formation presents a real formation water contamination problem when a known, highly effective lixiviant comprised of an aqueous solution of ammonium carbonate and/or bicarbonate is used to leach the desired values from the formation. Here, the ammonium ions from the lixiviant are exchanged into the smectite clays in the formation which make their removal by flushing with ground water a very slow and extended process.
One method for removing ammonium ions from a formation following a uranium leach operation is disclosed in U.S. Pat. No. 4,079,783, issued Mar. 21, 1978, and in copending U.S. application Ser. No. 824,686, filed Aug. 15, 1977, wherein a restoration fluid comprising an aqueous solution of a strong, soluble, alkaline compound is flowed through the formation to convert ammonium ions to an un-ionized form, i.e., ammonia (NH.sub.3), which, in turn, can easily be flushed from the formation. However, while this approach achieves a good result, it requires a substantial amount of alkaline compound, e.g., lime or sodium hydroxide, and produces a large quantity of waste water containing calcium and ammonia which has to be properly disposed of at the surface.
Further, the ion exchange between the ammonium ions and the cations in the aqueous alkaline solution takes place according to the mass action law. Consequently, the rate of ammonium ion removal becomes slower and slower as more and more of the ammonium ions are removed. This makes the last or residual ammonium ions very difficult to remove. In order to meet certain governmental requirements (e.g., Texas requires no more than 3 parts per million level of ammonia in the formation water), it has been estimated that approximately 99.5 percent of the ammonium ions in a contaminated clay formation has to be exchanged by cations from the restoration fluid. This obviously requires a considerable amount of alkaline solution to be handled to restore the formation to the required specifications. Also, the ammonia content of the recovered restoration fluid requires this fluid to be treated to remove the ammonia before the fluid can be used to make up fresh restoration fluid for recycle or before it can otherwise be disposed of.